1. Field of the Invention
The present invention relates to a source follower, and more particularly to a semiconductor device having amplifiers such as source followers formed by using thin film transistors in a driver circuit.
It should be noted that a semiconductor device in this specification refers to all the devices which function by the use of semiconductor characteristics. For example, an electro-optical apparatus, a semiconductor circuit, and an electronic apparatus all fall in the category of the semiconductor device.
2. Description of the Related Art
FIG. 7 is a circuit diagram of a general source follower. In the source follower shown in FIG. 7, an input potential Vin is supplied to the gate (G) of a transistor 901 while a potential Vdd (Vdd>Gnd (ground potential)) is supplied to the drain (D) thereof. In addition, the source (S) of the transistor 901 is connected to a constant current source 902 and a potential of the source is equal to an output potential Vout.
The output potential Vout of the source follower having the above configuration is expressed by the following Formula 1. It should be noted that Vgs corresponds to a voltage (gate-source voltage) obtained by subtracting a source potential from a gate potential.Vout=Vin−Vgs  [Formula 1]
A value of the gate-source voltage Vgs is determined by the relationship among the input potential Vin, the potential Vdd, and a drain current Id. When the transistor 901 operates in a saturation region, the drain current Id is expressed by the following Formula 2. It should be noted that i denotes mobility, Co denotes a gate capacitance per unit area, W/L denotes a ratio of the channel width (W) to the channel length (L) in a channel forming region and Vth denotes a threshold voltage.Id=iCoW/L(Vgs−Vth)2/2  [Formula 2]
In Formula 2, each of the i, Co, W/L and Vth is a value determined by an individual transistor. The drain current Id of the transistor 901 is almost determined by the constant current source 902. Consequently, Formula 2 shows that when the threshold voltage Vth is at a constant value, a predetermined value of the gate-source voltage Vgs can be obtained. Conversely, when the threshold voltage Vth varies, the gate-source voltage Vgs also varies correspondingly, leading to variations in the output potential Vout.
As a conventional method for adjusting the output of source followers, there is the one providing an output correction circuit (see Japanese Patent Laid-Open No. Hei 11-073165).
Assumed now is a circuit having source followers. Generally, a source follower is required to have a circuit for generating a constant current. Therefore, in the simplest case, it is constructed by using one transistor whose source terminal is connected to a power supply line, and operating the transistor in a saturation region by supplying a constant potential to the gate terminal thereof. The constant current Id at this time can be expressed by the above Formula 2.
When a potential of the power supply line varies according to a flowing current, the gate-source voltage Vgs of the transistor also varies, leading to variations in the constant current between each source follower. As a result, the output of the source followers varies even with the same input potential as described above.
As a means to solve the problem, there is a method of constructing a correction circuit as disclosed in Japanese Patent Laid-Open No. Hei 11-073165. However, several signals are required additionally in this case. In addition, this method cannot be employed in the circuit which can hardly secure enough time for correction. Alternatively, a method of using a power supply line which is thick enough to maintain a potential of the power supply line constant, or a method of lowering a resistance of a wiring by thickening a film of the wiring can also be employed. However, these methods are disadvantageous in that the circuit scale becomes quite large.